The present invention concerns a method of reconstruction of a three-dimensional image of elements of sharp contrast, in particular, a prosthesis present in a patient""s artery, from a set of two-dimensional images of the patient obtained for different positions of a camera.
In the medical field, reconstruction of the patient""s internal structures under examination is undertaken, with reconstruction, in particular, of angiographic images, that is, obtaining images of opacified vascular systems by injection of a contrast medium.
The two-dimensional projected images of a patient are obtained by rotation of the X-ray camera around the patient. As is common in angiography. each acquired image is a subtracted image which, for example, is obtained by a standard technique of logarithmic subtraction of two X-rays taken at the same angle of incidence before and after an injection of a contrast medium into the vascular system whose three-dimensional image it is desired to reconstruct.
A three-dimensional reconstruction algorithm is then made from two-dimensional projected images of the object in order to reconstruct the three-dimensional volume.
There are several types of reconstruction algorithms in X-ray imagery. One can cite, notably, the article by Gordon, Bender and Herman entitled xe2x80x9cAlgebraic Reconstruction Techniques (ART) for Three-Dimensional Electron Microscopy and X-ray Photography,xe2x80x9d Journal Theo. Biol. 29, pages 471 to 481 (1970); the article xe2x80x9cPractical cone-beam algorithm,xe2x80x9d L. A. Feldkamp, L. C. Davis and J. W. Kress, Journal Optical Society of America, vol. 1, no. 6, June 1984, pages 612-619; as well as the work by Anil K. Jain entitled xe2x80x9cFundamentals of digital image processing,xe2x80x9d Prentice Hall Information and System Sciences Series, Thomas Kailath Series Editor; Sep. 1988 or also French Patent Applications No. 89 03606, No. 89 16906 or No. 9610774, to which the reader is invited to refer.
Now, such angiographic image reconstruction makes it possible to visualize solely the arteries in which an opaque medium has been introduced, it does not make possible the visualization of other elements of interest, such as prostheses.
The invention is intended to remedy this problem by proposing a method which permits the visualization of prostheses in three dimensions.
An embodiment of the invention permits the visualization of prostheses and arteries on the same screen according to their actual arrangement.
The invention therefore proposes a method of reconstruction of a three-dimensional image of elements of sharp contrast from a set of two-dimensional images of an object comprising the elements of sharp contrast. For each different position of an X-ray camera around the object, a two-dimensional image is taken, and the use of an algorithm for reconstruction of the three-dimensional image is preceded by a stage of filtering of the set of two-dimensional images.
The three-dimensional image obtained only contains the elements of sharp contrast.
The camera can supply only one series of two-dimensional images, in which case no opaque medium is used, but it can also supply two series of two-dimensional images obtained by two acquisition sequences intercalated with an injection of opaque medium into the arteries. The two series of images can be used for a possible angiographic reconstruction and the first series can be used for the reconstruction of a three-dimensional image of elements of sharp contrast according to the invention.
According to an advantageous characteristic of the invention, the filtering of each acquired two-dimensional image is of low-pass type, so as to obtain a filtered two-dimensional image no longer containing the elements of sharp contrast. After filtering, a logarithmic substraction is carried out between that image no longer containing the elements of sharp contrast and the acquired two-dimensional image, in order to maintain only the elements of sharp contrast.
More precisely low-pass filtering on an acquired two-dimensional image consists of:
taking the acquired two-dimensional image,
determining its average,
expanding the acquired two-dimensional image in order to eliminate undesired objects of sharp contrast and small size in relation to the elements of sharp contrast, and
taking the maximum between the acquired two-dimensional image expanded and the average of the acquired two-dimensional image, in order to obtain a filtered two-dimensional image lacking any element and object of sharp contrast.
According to an advantageous embodiment of the invention, the elements are prostheses such as coils placed in a patient""s artery or even vascular clips
Preferably, after the reconstruction of three-dimensional images containing prostheses, a hysteresis segmentation of that three-dimensional image is made, so as to eliminate unnecessary voxels and to solely visualize the prostheses in a primary three-dimensional image.
According to an advantageous variant of the invention, for each prosthesis:
a back projection of the primary three-dimensional image is made on the acquired two-dimensional images in order to determine the pixels representing the prosthesis on these images,
an approximation is made by replacing the intensity of each of those pixels with an intensity determined from the intensities of the neighboring pixels in order to obtain primary two-dimensional images lacking the prosthesis,
a logarithmic subtraction is made between the primary two-dimensional images lacking the prosthesis and the respective acquired two-dimensional images, so as to obtain new two-dimensional images solely containing the prosthesis, and
a three-dimensional image reconstruction is made, followed by an hysteresis segmentation from new two-dimensional images, in order to obtain a secondary three-dimensional image solely containing the prosthesis.
This variant advantageously makes it possible to obtain a secondary three-dimensional image in which the coils present more precise intensity levels or gray levels.
In an embodiment of the invention, an angiographic three-dimensional image reconstruction of the arteries is made independently and merging is carried out between the primary or secondary three-dimensional image containing the prosthesis and the three-dimensional image containing arteries so as to obtain a merged three-dimensional image showing the spatial distribution between the arteries and the prosthesis.
According to an advantageous embodiment, in the course of merging, when a voxel in the three-dimensional image containing the arteries has an intensity different from the one having the equivalent voxel in the three-dimensional image containing the prosthesis, the highest intensity is attributed to the voxel resulting from merging.
Advantageously, before merging of the primary or secondary three-dimensional image containing the prosthesis with the three-dimensional image containing the arteries, voxels having a low attenuation value are eliminated on the three-dimensional image containing the arteries by maintaining only a predetermined number of voxels of high attenuation value.
According to another advantageous variant of the invention, for each prosthesis:
a back projection of the primary three-dimensional image is made on the acquired two-dimensional images in order to determine pixels representing the prosthesis on those images,
an approximation is made by replacing the intensity of each of those pixels with an intensity determined from the intensities of the neighboring pixels in order to obtain primary two-dimensional images lacking the prosthesis,
a logarithmic subtraction is made between the primary two-dimensional images lacking the prosthesis and two-dimensional images containing the arteries and the prosthesis acquired after injection of the medium opaque to X-rays, so as to obtain new two-dimensional images containing the arteries and the prosthesis, and
a three-dimensional image reconstruction is made from the new two-dimensional images in order to obtain a tertiary three-dimensional image containing the arteries and the prosthesis.
The operation of merging two three-dimensional images is thus avoided.